Various freeze processes have been developed to produce potable water from seawater or brackish water; to concentrate fruit juices such as orange juice and grape juice, vegetable juices such as tomato juice, and coffee; and to separate dissolved or suspended salts from the liquid carrier. See, for example, the patents of Ashley et al U.S. Pat. No. 3,070,969; Ashley U.S. Pat. No. 3,477,241; Ashley U.S. Pat. No. 3,501,924; Ganiaris U.S. Pat No. 3,620,034; Johnson U.S. Pat. No. 3,664,145; and Ogman U.S. Pat. No. 4,091,635.
Many types of equipment and heat exchangers have been used in the described freeze processes, but shell and tube heat exchangers, although widely used for heat exchange, apparently have been used only on a limited basis as freeze exchangers.
A shell and tube heat exchanger has an array of tubes extending between and through two spaced apart tube sheets surrounded by a shell. The shell is provided with an inlet and an outlet so that a suitable heat exchange fluid can be circulated through the shell to cool or heat a fluid flowing through each tube.
Each end of the array of tubes can be left open, or exposed, for use in some processing operations. For other operations, one or both ends can be enclosed by a fluid retaining header or box, which may or may not have a removable cover or access port. When only one header is present it can be either a fluid inlet or fluid outlet header. When a header is positioned at each end, one header can be a fluid inlet while the other can be a fluid outlet. Such an arrangement is conventional for once-through or single pass heat exchangers. However, for split pass heat exchangers, one of the headers can be divided into separate inlet and outlet sections. The header at the other end can be used to conduct fluid exiting from the outlet end of one group of tubes to the inlet end of the other group of tubes. The fluid inlet and outlet headers, or portions thereof, are provided with suitable conduit means for supplying and removing fluid.
Although shell and tube heat exchangers are generally used to heat a fluid stream, they can be used for cooling such a stream. When used for cooling purposes, each tube outlet end can be left open or uncovered so that the effluent can exit without restriction into a suitable receptacle. Similarly, each tube inlet end can be left open and the fluid to be cooled fed to the tube by any suitable means. Thus, if a liquid is to be cooled, a weir can be provided around the tube sheet so that a pool of liquid is formed and flows into the open mouth of each tube. Of course, one or both ends of the tubes can communicate with a closed header.
Shell and tube heat exchangers of the described types can be used as freeze exchangers for producing ice from water, for producing fresh water from brackish water and seawater, for concentrating fruit and vegetable juices, and in industrial crystallization processes. As the liquid flows through each tube, it can be cooled enough to crystallize a solid from the liquid. Thus, by cooling seawater, ice is obtained which when separated, washed, and melted provides potable water. When a fruit or vegetable juice is similarly chilled, ice forms and is removed to provide a concentrated juice.
Heat exchangers of the described types can use any cooling fluid on the shell side to cool a liquid flowing through the tubes. The fluid can be fed through one end and removed through the other end of the heat exchanger in a substantially unidirectional flow. Some suitable cooling fluids are ethyleneglycol and vaporizing refrigerants like ammonia and the Freon type refrigerants.
One of the major problems in freeze concentrating a liquid mixture in a shell and tube freeze exchanger has been the deposition of frozen solvent (usually ice) or solute, on the freeze exchanger surfaces. Ice build-up is usually the result of one or more of the following: dull or rough surface; surface flaws; coating discontinuities; corners; edges; non-uniform wetting of surface; low brine concentration; high heat flux; excessive ice fraction; inadequate flow rae; and local cold spots. Buildup of a solid, such as ice, lowers heat transfer and reduces the efficiency of the apparatus. In addition, if buildup of ice or some other frozen solvent or solid continues it can plug the apparatus completely, making it necessary to shut down to thaw the frozen material. This is true whether tubes or other shaped structures define the passages through which the liquid flows, such as a plate heat exchanger. Accordingly, a need exists for improved heat exchangers which can be used over extended periods of time by avoiding formation of solids deposited on each passage wall. Also needed are novel methods of freeze concentrating liquids which avoid or minimize formation of solid deposits on the fluid passage walls.